Shielded cords



Sept. 20, 1966 F. E. TIMMONS SHIELDED CORDS Filed May a, 1964 INVENTOR. 529/716 5 77/27/2700.?

I AY 2.0 21193.

United States Patent Ofitice 3,274,329 Patented Sept. 20, 1966 This application is a continuation-inpart of copending application, Serial No. 273,697, filed on April 17, 1963, and now abandoned.

This invention relates to electrical cords and more particularly to an electrical cord including at least one conductor provided with a spiraled shield.

The use of shielded type cords has become increasingly more prevalent, and such cords are incorporated in numerous electrical measuring and communication systems where a high signal-to-noise ratio is desirable. For example, shielded cords are commonly utilized on telephone handsets, ras microphone cables for radio transceivers, as probe cables for Geiger counter equipment, etc. These cords generally employ one or more insulated signal carrying wires which have suitable electromagnetic and/ or electrostatic shielding. The function of the shielding is to preclude extraneous voltages or signals from being picked up and adversely affecting the signal carried by one or more of the wires contained in the conductor and/or otherwise inhibiting the transmission of signals by the shielded wires.

Shielding 'has conventionally been in the form of metallic braiding disposed about the conductor or an insulated metal foil spirally wrapped about the conductor, Metallic braiding is relatively expensive and the use of metallic braiding leads to a diameter buildup larger than is desirable in certain applications. Spirally wrapped insulated foil, while being relatively inexpensive shielding, has previously limited the range of frequencies which could be handled by a conductor shielded with the same.

A further problem that has arisen in connection with various braided and non-braided forms of shielding previously employed has been the spurious noise voltages that are internally generated when the cord is subjected to normal flexing, such as that caused by vibrations of associated equipment or that resulting from expansion and contraction of a retractile or coiled cord incorporating the shielded conductor. Such internally generated noise voltages often result in distortion of the signal or signals beingcarried by the shielded wire.

It is the prime object of the present invention to provide an improved shielded cord construction. Another object of the invention is the provision of a shield for a conductor which permits relatively wide ranges of frequencies to be carried by the conductor. A further object resides in the provision of a retractile cord shielded with a spirally wrapped flexible metallic medium that possesses the desirable properties of extensibility and retractability. Still another object is to provide a retractile cord including at least one insulated wire shielded with a spirally wrapped flexible metallized medium that can withstand numerous extension and retraction cycles absent the accompanying generation of substantial spurious noise voltages.

Other objects and advantages of the present invention will become apparent from the following description thereof when considered in conjunction with the accompanying drawings wherein:

FIGURE 1 is a perspective view of an extensible and retractable coiled cord embodying various features of the present invention;

FIGURE 2 is a substantially enlarged perspective view partially broken away of one conductor of the coiled cord shown in FIGURE 1 provided with the shielding of the present invention; and

FIGURE 3 is an enlarged cross-sectional view of the retractile coiled cord shown in FIGURE 1 further illustrating the shielding of the present invention.

In general, the cord depicted in the drawings includes an outer insulating sheath, having at least one insulated wire confined therein which is provided with a spiraled shield that provides complete shielding coverage for the wire. The shield employs dual superimposed spirally wrapped tapes that are ribbon-like strands of insulated metallic foil. The tapes are wrapped about the insulated wire with the metallic surfaces thereof facing each other, and with a drain wire disposed therebetween.

For purposes of explanation, the invention will be described in connection with a coiled or retractile cord. While the invention offers definite advantages in any shielded cord, as described hereinafter, additional advantages of the invention are obtained in its use in a retractile cord.

Referring in particular to FIGURE 1, there is disclosed one form of retractile cord 10 which includes a plurality of insulated wires 11, at least one of which is provided with a spiraled metallic shield 12 of the type contemplated by the present invention. The coiled cord 10 is formed with a plurality of helical turns or convolutions 10a having a substantially uniform diameter. Oppositely disposed substantially straight end portions 10b and of the cord 10 extend tangentially from the coiled central portion and act as terminals for the cord. The cord 10 includes an outer insulating sheath or jacket 13 which is fabricated of a material such as an extruded synthetic rubber. A thin coating 14 of a protective tissuelike material, prefer-ably composed of cellulosic fibers, is provided along the inner peripheral surface 13a of the sheath. This coating serves to protect the wires 11 during the extrusion of the outer insulating sheath 13 thereabout, as hereinafter described.

Each of the individual wires 11 preferably includes a stranded central conductor 15 and an insulating sleeve 16 that is formed of a material such as rubber, plastic, etc. These individual wires 11, including the wire 11 provided with the spiraled shield 12, are preferably wound about each other in spiraled fashion to provide a compact cord.

The spiraled shield 12, which is provided about only one of the Wires 11 in the illustrated embodiment, serves to isolate the wire provided therewith from stray electrical signals that may distort the signal being carried by this wire. Further, the shielding inhibits radiation of signals by the wire. The nature of this shielding 12 and the formation thereof so that effective shielding is carried out thereby without an accompanying high degree of internally generated noise will best be understood from a consideration of FIGURES 2 and 3.

As shown in these figures, the shielding 12 includes an inner spiraled tape 18 which is a relatively thin metallized strand of flexible material and an outer spiraled tape 19 of similar construction. The term metallized as used herein is intended to be sufiiciently broad so as to include laminated metal to insulated tape film as well as evaporated, sprayed and other types of metal coatings on flexible insulated film. The tape 18 is spirally served or wrapped about the wire 11 to provide complete coverage therefor. That is, the shielding tape 18 is wrapped with the flexible insulating base layer 18a thereof maintained in contact with the wire insulating sleeve 16 and a layer 18b of a conductive shielding material, which is intimately bonded to the base layer, facing outwardly. The tape 19 is similarly spirally wrapped or served about the tape 18; however, as shown, this outer shielding tape is wrapped so that the base and conductive layer 19a and 1%, respectively, are disposed oppositely to those of the tape 18.

It should be understood that the dual metallized flexible tapes 18 and 19 must be sufliciently thin to preclude substantial diameter buildup in a coiled cord incorporating one or more wires shielded thereby. Moreover, the shielding material must be sufliciently strong to withstand the tensile stress imparted thereto during normal use of the cord. In addition, the metallized strands should be capable of withstanding numerous expansion and retraction cycles of a coiled cord employing same without the continuity of the metallized coating being destroyed.

These results are achieved by employing base layers for each of the tapes 18 and 19 which are preferably strands or ribbons of a suitable flexible material such as polyethylene terephthalate, polytetrafluorethylene, or polychlorotrifluoroethylene, which materials are commercially available under the trademarks Mylar, Teflon and Kel-F, respectively. Other insulated films having the desired properties of strength and flexibility such as propylene, etc., may also be used. Mylar is the more preferable base material for use in the shield 12 employed in the retractile cord since it is relatively inexpensive and has high tensile strength in small cross section. Preferably, the layer of conductive shielding material which is intimately bonded to the base layer of each of the shielding tapes 18 and 19 is a thin coating of a conductive material such as aluminum, silver, copper, etc.

The inner tape 18, which is spirally Wrapped about the conductor 11 and maintained in contact with the insulated sleeve 16 thereof, is formed with a width suflicient to allow one longitudinal edge portion thereof to overlap the oppositely disposed edge portion along the entire length of the wire 11. Similarly, the outer shielding tape 19, which has the metallized coating 1% facing inwardly and is maintained in the intimate contact with the metallized layer 18b of the inner tape 18, is also spirally served with overlapping edge portions.

The turns of the outer tape are perferably offset relative to the turns of the inner tape so that the tapes short circuit each other, that is, the tapes bridge the gaps or insulated edges between turns. Thus series inductance resulting from the turns is minimized thereby permitting the conduct-or to carry a wide range of frequencies. The insulated conductor is effectively surrounded by two conductive tubes or pipes. In addition, a drain or ground wire 20 is confined between and maintained in electrical contact with the metallic coatings on the spirally served inner and outer tapes 18 and 19. This ground wire 20 is preferably spirally Wrapped along the length of the shielded conductor 11 in complementary fashion with the serving of the inner and outer tapes 18 and 19. The ground wire may be formed of a single or of multiple conductive strands. When the ground wire is formed of a plurality of strands, such strands may be spiralled together to form a compact circular assembly as shown in FIGURE 2, or they may be arranged in close parallel fashion so as to form in effect a generally flat ground wire. The ground wire provides a low resist-ant path for any currents induced in the tapes 18 and 19 and thus serves as a convenient and suitable means for grounding or terminating the shield.

An outer layer or insulating film 21 is spirally wrapped about the entire shield 12. Preferably, this insulating film 21 is formed of the same material as are the flexible base layers 18a and 19a of the shielding tapes 18 and 19, and the film is firmly wrapped thereabout. The film 21 serves to minimize movement of the shielding tapes relative to each other as the coiled cord is subjected to numerous expansion and retraction cycles and enhances the flexing life of the spiraled shield 12.

The spirally served dual tapes 18 and 19, as described above, not only serve as an effective electrostatic and electromagnetic shield but also minimize internal noise generation which can severely limit the utility of shielded retractile cords. In this connection, when two surfaces of materials having dissimilar dielectric characteristics are brought into intermittent contact, there is a separation and buildup of dissimilar charged particles on the contacting surfaces of the materials. Accordingly, when a metallic shield is maintained in contact with the insulated sleeve of a signal carrying wire, a similar accumulation of charged particles results. A capacitive effect stems from this buildup of dissimilar charged particles and leads to extraneous signals being supplied to the signal carrying wire as a result of adjustments in the distribution of the dissimilar charged particles. These spurious signals are the primary source of noise in shielded cords.

However, the shielding of the present invention is such that the dielectric characteristics of the innermost surface of the dual tape shield 12 are not substantially dissimilar from those of the insulating sleeve 16 of the wire 11. Accordingly, during flexing of the coiled cord 10 including the spirally served shielding tapes 18 and 19, no substantial capacitive effect is realized at the surface of the wire 11. Similarly, since the metallic layers which are bonded to the flexible base layers are maintained in intimate electrical contact and have substantially identical dielectric characteristics, the buildup of dissimilar charges in the regions between these metallized shielding wires is precluded.

An extensible and retractable coiled cord including at least one insulated wire having a relatively thin dual tape shield spirally Wrapped thereabout can be produced in any desired length in accordance with conventional coiled cord producing methods. That is, the several interwound conductive members or wires 11 are enclosed by the extruded outer sheath or jacket 13, and the entire cord is helically wound about a mandrel with the ends thereof remaining substantially straight to provide the terminal portions 101) and 100. Thereafter, the mandrel together with the cord wrapped in helical fashion thereabout is subjected to successive heating and cooling steps which effect a setting of the sheath in coiled form. Preferably, the coiled cord is worked after cooling to reverse the direction of the convolutions 10a and thereby increase the tendency of the cord to remain in a contracted state.

In one specific embodiment of the invention, the metallized tapes 18 and 19 have a width of approximately and a thickness of approximately .001". In this connection, the flexible base layer of each is preferably .0005" in thickness and the metallized coating deposited thereon has a thickness of .00035". The tapes so constructed were spirally served in the superimposed relationship, as hereinbefore described, to provide complete shielding for the wire 11 provided therewith.

Various features of the invention are set forth in the accompanying claims.

\Vhat is claimed is:

1. A shielded cord comprising at least one insulated wire, dual superimposed tapes spirally served about said insulated wire, said tapes each being served with longitudinal edge portions thereof overlapping and including an insulating layer and a metallic layer, the metallic layers being faced toward each other and an uninsulated drain wire disposed between said superimposed tapes in contact with both of said facing metallic layers.

2. In an extensible and retractable coiled cord that includes an outer insulating sheath formed with a plurality of helical convolutions and having a plurality of insulated wires disposed within the sheath and extending along the length thereof, a flexible electrostatic and electromagnetic shield provided about at least one of said insulated wires, which shield comprises a pair of dual superimposed inner and outer tapes of flexible insulating material of relatively high tensile strength having metallized coatings intimately bonded thereto, said inner tape being spirally served about said insulated wire with longitudinal edge portions thereof overlapping and with the metallized coating outward and said outer tape being spirally served about said inner tape with longitudinal edge portions thereof overlapping and with the metallized coating inward and maintained in contact with the metallized coating of said inner tape, and an uninsulated grounding wire maintained between said inner and outer tapes along the length of said shield and in contact with said metallized coatings.

3. An extensible and retractable coiled cord which comprises an outer sheath having a plurality of helical convolutions; at least one insulated wire provided with insulation having predetermined dielectric characteristics disposed within said sheath .and extending along the length thereof; an electrostatic and electromagnetic shield spirally wrapped about said insulated wire, said shield including a pair of dual superimposed inner and outer tapes of flexible insulating material having similar dielectric characteristics as said insulation and having metallized coatings intimately bonded thereto, said inner tape being spirally served about said insulated wire with longitudinal edge portions thereof overlapping and with the metallized coating outward, said outer tape being spirally served about said inner tape with longitudinal edge portions thereof overlapping and with the metallized coating inward and maintained in contact with the metallized coating of said inner tape; and an uninsulated grounding wire maintained between said inner and outer tapes and extending along the length of said shield.

4. An extensible and retractable coiled cord which comprises an outer sheath having a plurality of helical convolutions; at least one insulated wire disposed within said sheath and extending along the length thereof; an electrostatic and electromagnetic shield spirally wrapped about said insulated wire, said shield including a .pair of dual superimposed inner and outer tapes of polyethylene terepht-halate having metallized coatings intimately bonded thereto, said inner tape being spirally served about said insulated wire with longitudinal edge port-ions thereof overlapping and with the metallized coating outward, said outer tape being spirally served about said inner tape with longitudinal edge portions thereof overlapping and with the metallized coating inward and maintained in contact with the metallized coating of said inner tape,

said metallized coating having similar dielectric characteristics; an uninsulated grounding wire maintained between said inner and outer tapes in contact with said inward facing metallized coatings and extending along the length of said shield; and an insulating film formed of a flexible material firmly wrapped about said shield so as to preclude relative movement between said spirally wrapped inner and outer tapes.

5. A shielded cord which comprises an outer sheath; at least one insulated wire disposed within said sheath and extending along the length thereof; an electrostatic and electromagnetic shield spirally wrapped about said insulated wire, said shield including a pair of dual superimposed inner and outer tapes of polyethylene terephthalate having metallized coatings intimately bonded thereto, said inner tape being spirally served about said insulated wire with longitudinal edge portions thereof overlapping and with the metallized coating outward, said outer tape being spirally served about said inner tape with its turns being offset relative to complementary turns of the inner tape with longitudinal edge portions thereof overlapping and with the metallized coating inward and maintained in contact with the metallized coating of said inner tape, said metallized coatings having similar dielectric characteristics; an uninsulated grounding wire maintained between said inner and outer tapes in con-tact with said inwardly facing metallized coatings and extending along the length of said shield; and an insulating film formed of a flexible material firmly wrapped about said shield so as to preclude relative movement between said spirally wrapped inner and outer tapes.

References Cited by the Examiner UNITED STATES PATENTS 2,344,501 3/1944 Bennett 17436 2,764,625 9/1956 Ingmanson 174--69 3,032,604 5/1962 Timrnons 174-36 X LEWIS H. MYERS, Primary Examiner.

JOHN F. BURNS, Examiner.

D. A. KETTLESTRINGS, Assistant Examiner. 

1. A SHIELDED CORD COMPRISING AT LEAST ONE INSULATED WIRE, DUAL SUPERIMPOSED TAPES SPIRALLY SERVED ABOUT SAID INSULATED WIRE, SAID TAPES EACH BEING SERVED WITH LONGITUDINAL EDGE PORTIONS THEREOF OVERLAPPING AND INCLUDING AN INSULATING LAYER AND A METALLIC LAYER, THE METALLIC LAYERS BEING FACED TOWARD EACH OTHER AND AN UNINSULATED DRAIN WIRE DISPOSED BETWEEN SAID SUPERIMPOSED TAPES IN CONTACT WITH BOTH OF SAID FACING METALLIC LAYERS. 